A typical semiconductor device includes a wiring substrate and a semiconductor chip mounted on the wiring substrate. In the wiring substrate, for example, wiring layers and insulation layers are alternately stacked on a substrate body, which is composed of silicon. A via extends through each insulation layer to connect the adjacent upper and lower wiring layers. Japanese Laid-Open Patent Publication Nos. 2005-95977 and 2013-93547 describe examples of conventional semiconductor devices.
In a semiconductor device, for example, a connection pad is arranged on an outermost layer of a wiring substrate. A joining member such as a solder bump electrically connects the connection pad to an electrode pad of the semiconductor chip. Due to environmental concerns, lead-free solder, which does not include lead (Pb), is used as the joining member. For example, Sn—Bi solder, which has a relatively low melting point, may be used as the lead-free solder.
One example of a method for manufacturing a semiconductor device will now be described.
Referring to FIG. 9A, a solder bump 72 is formed on a connection pad 70P of a wiring substrate 70. For example, Sn-57Bi (melting point: 137° C.) is used as the solder bump 72. Then, the distal end of a connection terminal 81, which is formed on an electrode pad (not illustrated) of the semiconductor chip 80, contacts the solder bump 72. This arranges the semiconductor chip 80 on the wiring substrate 70 with the solder bump 72 located in between. The bump 72 is heated and melted in a reflow furnace. Then, the melted bump is solidified. As illustrated in FIG. 9B, this electrically connects the connection pad 70P of the wiring substrate 70 to the electrode pad of the semiconductor chip 80 via the solder bump 72 and the connection terminal 81.
When the solder bump 72 joins the connection pad 70P and the connection terminal 81, intermetallic compound layers 73 and 74 are formed at the boundary between the connection pad 70P and the solder bump 72, as illustrated in FIG. 9B. Further, intermetallic compound layers 75 and 76 are formed at the boundary between the connection terminal 81 and the solder bump 72. When the solder bump 72 has an Sn—Bi composition and the material of the connection pad 70P and the connection terminal 81 is copper (Cu), the intermetallic compound layers 73 and 75 are composed of an intermetallic compound of Cu3Sn, and the intermetallic compound layers 74 and 76 are composed of an intermetallic compound of Cu6Sn5.